This invention relates to rotary drill bits for drilling well bores in the earth, and more particularly to rotary drill bits having both conical roller cutters and drag cutters.
This invention involves an improvement over combination rotary drill bits of the type, such as shown, for example, in U.S. Pat. Nos. 4,006,788 and 4,285,409, comprising a plurality of rotatably mounted roller cutters each having a generally conical roller cutter body and a plurality of cutting elements on the body, and a plurality of drag cutting elements mounted on supports extending down between the roller cutters. The cutting elements on the roller cutters "drill" the formation by crushing or fracturing it. In contrast, the drag cutting elements "drill" by shearing the formation, which offers faster rates of penetration than drilling by crushing for certain types of formation. Combination drill bits attempt to advantageously utilize the differences in cutting action of these two types of cutting elements by positioning the cutting elements on the bit in predetermined relation with respect to each other such that each type of cutting element performs the cutting function for which it is best suited. For example, in U.S. Pat. Nos. 4,006,788 and 4,285,409, the drag cutting elements are so positioned relative to the cutting elements on the roller cutters that the cutting edges of the drag elements are at or slightly below the bottom of the paths followed by the tips of the cutting elements on the roller cutters. This arrangement of cutting elements enables both types of cutting elements to be supported in drilling engagement with well bore bottom. In addition, because the drag elements are of a diamond material which is subject to rapid deterioration upon excessive heat build-up, this arrangement also serves to extend the useful life of the drag elements by limiting their penetration into the formation, the amount of formation they remove, and thus the amount of heat generated at their cutting edges.
However, these conventional combination drill bits have not proven to be entirely satisfactory in that their drag cutting elements still experience excessive heat build-up and thus have shortened useful lives, particularly when the bit is used to drill certain types of formations. This heat build-up is believed to be due to the fact that the drag cutting elements are in substantially continuous cutting engagement with the formation. Thus heat is continuously generated at the cutting edges of the drag elements. and, at the same time, the cutting edges of the elements at no time are exposed to the drilling fluid so as to be washed and cooled by the relatively cool drilling fluid as it flows over the well bore bottom. The problem of overheating due to continuous cutting engagement is particularly critical in drilling relatively plastically deformable formations which the drag cutting elements penetrate relatively deeply. Many commonly encountered formations, such as salts, shales, limestones, sandstones, and chalks, become plastically deformable under so-called differential pressure conditions, when the hydrostatic pressure of the column of drilling fluid bearing on the well bore bottom exceeds the pore pressure of the formation surrounding the bore, as often occurs in deep hole drilling.